Not a turn-friendly zone

What does San Francisco have against left turns? It seems like every street we'd cross while we were there prohibited left-hand turns. I understand the ones that are one-way streets, and I appreciate the sign helping me not get killed by an oncoming bus. But what about all of the other streets? What's their excuse? It's like this city just has something against ambi-turners. You'd think that a city that's so accepting of people swinging both ways would be more tolerant of those of us who like to turn both ways.